What was found
ON C/O, CAPTAINS EFIS IND SPEED TRIM WARNING LIGHT. A/C COULD NOT ATTAIN FL 290 IN MODE CONTROL PANEL, A/C DESCENDED & AIRSPEED DECREASED AS STAB OUT OF TRIM LIGHT ILLUM. TURNED OFF AUTOPILOT, STAB WOULD NOT TRIM IN ELECTRIC OR MANUAL MODE. LOWER ALTITUDE & SPEED TRIM SYS OPS NORM, TURNED BACK & LANDED WITHOUT EVENT. MX R & R AUTOPILOT STAB TRIM MOTOR & STAB TRIM G/B ASSY. HOR STAB TRIM CONTROL ADJUST & TEST ACCOMPLISHED. TEARDOWN OF STAB TRIM ACT FOUND WORN RINGS, SEALS, O-RINGS, UMBRELLA, BEARINGS, BRAKE PLATES & SHOES, SPRINGS, AND WIPERS. UNIT OVERHAULED IAW CMM. CONFIRMED FAILURE OF AUTOPILOT JACKSCREW SERVO MOTOR FOR NO OUTPUT SHAFT ROTATION DUE TO GEAR, PINION, & MOTOR ROTOR DAMAGED.